In the high volume fabrication of semiconductor integrated circuit devices, the technique of rapid thermal processing (RTP) or rapid thermal annealing (RTA) has become an important processing step in the fabrication of IC devices. In a conventional RTP process, a workpiece is heated by a heat source such as a plurality of tungsten-halogen lamps or arc lamps which provides almost instant heating effect on a workpiece such as a semiconducting substrate in the shape of a wafer. In most RTP methods, the heat treatment of a wafer takes place in a single process chamber with the appropriate process gas flow and composition.
A typical single-wafer RTP chamber 10 is shown in FIG. 1A. In RTP chamber 10, an outer chamber wall 12 made of metal is cooled by ambient air and liquid circulating in cooling channels 14. The wafer 18 and the wafer supports 20 are situated inside a fused silica inner chamber wall 24 equipped with a process gas inlet 26 and outlet 28. The wafer 18 supported by the fused silica supports 20 is heated radiatively by banks of lamps 22 of either tungsten-halogen or arc-type lamps.
The mode of heating provided by the RTP chamber 10 shown in FIG. 1A is dynamic in that the wafer never reaches thermal equilibrium with the heating elements. As a result, the temperature uniformity over the wafer surface depends on the heating rate of the wafer. Furthermore, the radiative coupling between the wafer and the heating lamps varies greatly with temperature due to the fact that the emissivity of silicon depends strongly on temperature up to about 700.degree. C. In addition, the radiative coupling depends on the physical state of the wafer front and back surfaces. During a typical RTP process as that shown in FIG. 1A, the wafer 18 is heated on both the top and the bottom surfaces and heat from the wafer radiates to cold wall surfaces. Based on this simultaneous heating and cooling, the rate of wafer heating and the wafer final temperature depend strongly on the wafer emissivity, which is in turn a strong function of the wafer surface structure, the wafer backside textures film stack, and wafer temperature.
Another conventional lamp based RTP chamber 30 is shown in FIG. 1B. In this chamber design, a semiconductor wafer 32 to be processed is placed on a susceptor 34 which can be raised up or down by an elevator 36. The lamp heaters 40 which are enclosed in a reflective dome 42 direct radiative energy toward the wafer 44. Process gases are pumped into chamber 46 through gas inlet 48 and exhausted through outlet 52. It should be noted that, unlike the chamber construction shown in FIG. 1A, the susceptor 34 is heated simultaneously with the wafer and aids in achieving temperature uniformity across the workpiece.
The present trends in semiconductor manufacturing indicate that in the near future the wafer heating rate and cooling rate will increase and that the time interval during which a wafer is maintained at a desired peak temperature will decrease substantially. The trends are dictated by high performance logic circuits that are based on very shallow junctions that must be prepared in a fabrication process with stringent thermal budget limitations. Moreover, in today's highly competitive environment, the manufacturing of semiconductor devices continues to strive for efficiency and throughput gains and thus, a more uniform and higher throughput RTP process is desired.
Conventional RTP chambers such as those shown in FIGS. 1A and 1B have performance limitations that limit their ability to meet future requirements of significantly faster heating rates. On the one hand, from a performance consideration, the ramp rate of conventional RTP chambers is limited by the requirement of temperature uniformity in the highly transient environment of conventional RTP chambers, which require feed-back control and independently addressable lamps in order to achieve an acceptable degree of temperature uniformity across the wafer workpiece. On the other hand, at the outer limits of tool performance the rate at which the wafer of a given size can be heated is limited by the time it takes for the lamp filament to achieve operational temperature and furthermore, by the radiant flux at the wafer. The latter is limited by how closely the heating lamps can be stacked together and the maximum radiant power flux of each lamp which is limited by the melting point of the tungsten filament and the softening point of the lamp's conventional fused silica enclosure. It has been found that the maximum achievable heating rate for an industry standard silicon wafer of 200 mm diameter in a conventional, lamp-based RTP chamber is limited to about 150.degree. C./sec.
In recent years, RTP has also been conducted in vertical hot wall furnace-type chambers in limited applications. One of such devices is shown in FIG. 1C. A hot wall RTP furnace 50 is closed on all but one side (the bottom) through which wafers 54 are introduced and subsequently removed from the furnace upon completion of the thermal cycling treatment. The hot wall RTP furnace 50 has a vertical axis along which the wafers move by the action of elevator 66 on a wafer carrier 46 while maintaining the plane of the wafers perpendicular to the vertical axis of the chamber 50. The furnace 50 is closed at the top and is equipped with a top heater 58, and closed on the sides where side upper heaters 62 are mounted thereto. The furnace 50 has a chamber 64 defined by a fused silica chamber wall 76 and may include additional heating zones 68, each of which is maintained at a specific temperature. The wafers 54, positioned on wafer carrier 46 can be moved into or out of the chamber 64 by an elevator 66. The chamber 64 is further heated by lower side heaters 68 to facilitate the control of chamber temperature. Various process gases may enter the chamber 64 through gas inlet 72 and be exhausted from the chamber through gas outlet 74. During operation, wafers 54 are slowly transported vertically through one or more temperature zones coming to a full stop at a desirable location where the wafers achieve a temperature that is much less (100-500.degree. C.) than that of the surrounding hot walls. After a suitable amount of time which may vary from several seconds to several minutes, the wafers can be withdrawn and allowed to cool. Limited by its basic design, the existing hot wall RTP furnace cannot achieve very high heating rates and short dwell times at a desirable wafer temperature.
Neither the lamp-based nor the hot wall-based RTP furnaces that are presently available can be used to process large workpieces (or wafers) at heating rates that are much higher than 150.degree. C./sec and at dwell times at peak temperatures that are much shorter than one second. Therefore, prevailing art in current practice cannot meet the processing requirements for high performance logic circuits.
It is therefore an object of the present invention to provide a method and an apparatus for RTP that does not have the drawbacks and shortcomings of conventional lamp-based or hot wallbased RTP furnaces.
It is another object of the present invention to provide a method and an apparatus for RTP in which the temperature of at least one workpiece can be increased and decreased uniformly at rates that are much greater than 150.degree. C./sec.
It is a further object of the present invention to provide a method and an apparatus for RTP which can be used to provide similar heating and cooling rates to wafers that are of common industrial sizes of 125 mm, 200 mm, 250 mm or 300 mm in diameter.
It is yet another object of the present invention to provide a method and an apparatus for RTP in which at least one workpiece can be rapidly heated, in the presence of processing gases or vacuum, by a single heat pulse of short temporal duration.
It is still another object of the present invention to provide a method and an apparatus for RTP in which the temperature of at least one workpiece can be maintained substantially uniform during rapid temperature cycling.
It is still another further object of the present invention to provide an apparatus for RTP which has a curved, horizontally oriented cavity structure that supports a plurality of zones with walls that are maintained at working temperatures.
It is yet another further object of the present invention to provide a method and an apparatus for RTP which utilizes a transport mechanism that is capable of carrying one or more workpieces unidirectionally through a horizontal processing chamber within a curved or linear cavity.